1. Field of the Invention
The present invention relates to a method for bending a pipe formed by welding and to a mandrel assembly for use in being such a pipe.
2. Description of the Related Art
A hollow workpiece such as a pipe is typically bent with a mandrel inserted thereinto to provide the hollow workpiece with a bent portion free from such deformation as buckling and with a constant inner diameter throughout the length thereof. However, from time to time, the insertion of the mandrel does not provide sufficient effect upon bending of the hollow workpiece. As a result, the hollow workpiece undesirably includes wrinkly deformation at the bent portion thereof.
To overcome this problem, there has been proposed a hollow workpiece bending apparatus as disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI-6-339731.
A hollow workpiece to be bent in such an apparatus is a seamless pipe of aluminum. A mandrel to be inserted into such a pipe is made of nylon and is sized to fit into the pipe. The mandrel inserted within the pipe extends forwardly through a workpiece-introducing aperture of a stationary die into a workpiece-introducing aperture of a movable die provided forwardly of the stationary die, such that a front end thereof is positioned at a central portion of the workpiece-introducing aperture. One supporting bar is connected to a rear end of the mandrel in support of the same.
Since the mandrel is an extruded product of nylon and extends into the workpiece-introducing aperture of the movable die, the hollow workpiece includes a portion to be bent which is supported by the mandrel inserted therewithin. Thus, it becomes possible for the bending apparatus to bend such a portion of the hollow workpiece without causing undesirable deformation such as buckle thereon. This bending apparatus is useful especially in providing a product including a bent portion of small radius or curvature.
However, the mandrel used in the aforementioned bending apparatus is designed to fit into the extruded hollow workpiece. Therefore, such a mandrel is less applicable to a weld tube having a bead including a projection or bulged portion protruding inwardly of the weld tube. If the mandrel is made thinner in correspondence to the bulged portion, the pipe inevitably includes very large gaps between a surface of the mandrel and inner surfaces thereof adjacent the surface of the bulged portion. This makes it difficult to precisely bend the pipe. Alternatively, one may propose provision of the mandrel with a recessed portion formed in conformation to the bulged portion, however, such a mandrel would become expensive.